The useful lifetime of a capacitive power storage device is affected by the voltage level stored on the capacitor(s) of the device. If capacitors in a series circuit configuration are not voltage balanced, some may wear out sooner than others. FIG. 1 shows a conventional circuit to balance (equalize) the voltage across capacitors C1 and C2 in series.
A disadvantage of this circuit is that resistor RB has to be selected to provide significant current draw IB1 and IB2 in order to achieve acceptable voltage equalization. However, a small value for RB causes larger power consumption.
Another disadvantage of this circuit is that the leakage current IL1 and IL2 of each capacitor is influenced by temperature, making it difficult to determine the difference between each capacitor's leakage current. Another disadvantage is that leakage current increases as the capacitors age, making the circuit less and less effective with time. Yet another disadvantage is that it takes a long time to balance the capacitor voltages. This reduces the useful life time of the capacitors especially in temperature environments.
A circuit such as the one described in co-assigned application no. US 20120224445 functions better than the passive voltage balance circuit of FIG. 1. However, there are drawbacks to such a circuit. For example, it is difficult to eliminate the side effects of “firmware halts” of the circuit in systems utilizing the capacitors for power, especially when balancing is underway. Second, it is difficult to balance multiple capacitors in series because the software of said systems typically executes step by step, instead of in parallel as the balancing hardware does.